Bonded product and method of making



Patented Apr. 16, 1940 2,196,972 BONDED PRODUCT AND METHOD OF MAKING Willis A. Boughton, Cambridge, and William R. Mansfield, Boston, Mass., assignors, by mesne assignments, to New England Mica Company. Waltham, Mass., a corporation of Massachusetts No Drawing. Application August 8, 1938,

Serial No. 223,719

22 Claims.

This invention relates to improvements in the manufacture of bonded products the materials of which have been bonded with bonding compounds that are subjected to high temperatures to effect the bonding. Such compounds are particularly useful in the art of bonding, binding, or cementing surfaces of particles which are nonreacting therewith, and non-soluble therein, thereby effecting the integration of discrete particles of matter into practically unitary bodies.

In the following description of the invention disclosed herein. reference is made specifically to the manufacture of laminated mica insulation products, but it must be understod that the application of the properties, principles and methods disclosed herein may be made to other uses and materials, for example the impregnation and bonding into unitary products of inorganic fibrous materials such as asbestos, spun glass, slag wool, with or'without other heat-resisting matter. Other applications of the invention will be obvious to those skilled in arts where the invention may be applied.

One object of the invention is to produce bonded products the binders of which shall exhibit a thermal behavior such that they will flow at red heat, and on cooling form clear, hard, glass-like substances characterized by a high degree of adhesion to adjacent surfaces and resilient when in the form of a film between such surfaces; shall be essentially unaltered upon repeated subjection thereafter to temperatures of redness; shall possess high electrical insulation resistance values and dielectric strengths; and shall be non-charring, non-combustible, and fireproofing.

A further object is to produce bonded products embodying binders having properties which make them especially beneficial in the manufacture of laminated mica plate for use, primarily, as insulation in electric heater appliances, but also in various other types of electric devices.

A further object of the invention relates to the production of composite insulating materials bonded or impregnated with the fused reaction product of one or the other of the several binder compounds described herein.

Further objects of the invention will be apparent to those skilled in the related arts after reading this specification.

50 Composite high temperature-resistant mica plate has attained a position of distinct and increasing importance in the field of electrical inacting current leakage maxima requirements of many present-day electric heater appliances. These limitations or shortcomings, together with recent trends in the variety and operating requirements of electric heater appliances impelled the development of the improved binders and products described herein.

The trend in electric heater appliances to higher temperatures of operation, greater' heat-up speeds and thermostatic operation, together with the requirement of specific maximum current leakages to eliminate shock hazard, has placed an increasingly greater burden on the insulation. In order, to satisfy the assembly and operating conditions of the element units of electric heater appliances, the insulation must possess a, high degree of mechanical integration to permit free punching, notching, and element Winding; must be capable of repeated subjection to the operating temperatures of the appliance essentially without physical or chemical changes; and must exhibit an electrical insulation. resist ance sufficiently high to eliminate the possibility of shock hazard at the operating conditions of the appliance.

It is obviously highly desirable in the manufacture of high temperature-resistant bonded mica plate and other bonded mica products to subject the bonding material, or the mixed components which after fusion and reaction constitute the bonding agent, to a fusion temperature as near as practicable to the disintegration temperature of the mica films themselves, in order to obtain maximum fluxing reaction of the bonding material and resulting thermal resistance of the binder, and to obtain in the mica plate those qualities most essential in subsequent conditions of assembly, use and operation. Thus, when India mica (Muscovite) films are used, the temperature of manufacture should be in the general range of about 580 C. to about 650 C. (1076 F.-- 1202 F.) and with amber mica (Phlogopite) manufacturing temperatures up to about 875 C. (1607 F.) may be used.

Another essential property of any inorganic bonding agent in use as a binder for high temperature-resistant mica plate is that it must show a low thermal coefficient of expansion, not great- 1y different from that of mica itself, at all temperatures up to and including temperatures of redness. Otherwise, cracking, buckling-or disruption of the mica plate results when it is heated to or cooled under pressure from temperatures of redness, during manufacture. Therefore, when we mention a good mica plate or mica binder we mean, among others, that this criterion also has been observed.

Therefore, in a search for inorganic compositions of matter suitable for use as binders for mica films in the manufacture of high temperature-resistant bonded mica plates, we have investigated the properties of many compounds, including boron trioxlde, in various combinations. Prior attempts to use boron trioxlde either alone or in combination with lead salts have not been practical commercially. Such products have been found to have various deleterious or disadvantageous properties which have prevented them from attaining any considerable commercial use. Mica plates made with such bonding materials have been oifered to the electrical appliance manufacturers from time to time but their faults have been so obvious that they are of extremely limited, if any, use today.

In U. 8. Patent 1,578,812, Dawes and Boughton had stated that boron trioxlde alone was unsuited to the conditions required because of its high fluidity at red heat. On the other hand, in U. S. Patent 1,386,008, McCulloch stated that boron trioxide softens at moderately elevated temperatures of the order of 500 C. (932 F.) but does not become fluid except at temperatures several hundred degrees above this point. Although various melting points are given in the literature, generally 577 C. (1071 F.), we have found that boron trioxlde has no sharply defined melting point, but rather softens and then increases in fluidity as the temperature is increased. Our experiments have shown that even when subjected to temperatures well above its stated melting point, 577 C. (1071 F.), boron trioxlde exhibits a failure to fuse thoroughly to give a clear glasslike adhesive form on cooling, especially under the conditions of manufacture of high temperature-resistant bonded mica plate. This residual opacity involves inadequate adhesion and clarity of the binder and inadequate hardness and mechanical integration of the bonded product, causing such products to be unsuited to meet the requirements of heater appliance insulation described above. We have been able to overcome these disadvantageous properties by mixing with boron trioxlde certain other inorganic compounds in limited variety; and thus, upon application of heat as usual we have obtained highly improved products. The resulting glass-like substances formed at high temperatures differ from the true glasses in the following particulars:

(a) They are adhesive to mica when in a fused state, whereas all low-melting commercial glasses that we have been able to obtain show marked lack of the necessary adhesion.

(b) The thermal coefficients of expansion of the fused composition are close to that of mica, while those of the low-melting glasses are so different from that of mica that when fused in contact therewith and cooled the glass cracks, and the plate is correspondingly imperfect.

(0) Th bonding efficiency of the major consituent in the bonding composition is enhanced by the presence of the other components, whereas ordinary glasses appear to have no major component that is, alone, essentially a mica adhesive,

and no combination of components has been found in which any one factor has a noticeable effect of enhancing the adhesion of glass to mica, with the possible exception of fluorine compounds occasionally employed, which, however, in the common glasses tried fail to have sumcient effect to permit the use of glass as an efflcient mica binder.

(d) The flowing point of the melted binder is distinctly lower and is in the range of the decomposition temperature of mica itself, while the socalled low-melting glasses of commerce tried thus far still melt only at temperatures higher than the decomposition temperature of India mica itself.

In the construction of the mica plate, we may apply the binder composition which may consist of the unfused mixed components or which may be a pre-fused reaction product, between the mica films as a dry powder, as a paste, in aqueous solution, or dispersed or dissolved in an aqueous or organic solvent or mixture of solvents. Thereafter the mica plate so constructed is heated, and the fusion reaction effected in situ producing the reaction product bonding agent during the manufacture of the bonded plate.

In manufacturing the high temperature mica plate the assembly constructed as above is, as a preliminary step, first treated for removal of the solvent or dispersing medium by heating to a suitable temperature, ordinarily 60 C.-70 C. (140 F.-158 F.), under reduced pressure in a vacuum oven. This removal of the solvent or suspending medium, partial if aqueous and complete if organic, is preparatory to the final fusion reaction of the bonding composition used. The degree of vacuum and the heat treatment must be so related as to liberate the vapors in such a gentle way that the structure of the mica plate, i. e. the overlappingrelation and flat position of its films remains unchanged at the end of this treatment. Some water may remain without being deleterious to the final fusion reaction but when an organic dispersing medium is used the solvent should be entirely removed at this stage in order to prevent the later formation of a charred residue, such as yielded by glycerine and its compounds, in the fused mica plate.

After this treatment the mica plate is subjected to temperatures of redness and to suitable compression to effect the chemical and physical changes necessary to produce the mechanical integration, thermal stability and other properties required of high-temperature-resistant mica plate.

The improved binders of this invention are based on the modification and improvement of the properties as a mica binder shown by boron trioxlde alone obtained by the incorporation into the original bonding mixture or solution or dispersion, of a fluorine-containing component which we have discovered has an extraordinary fiuxing effect on the boron trioxlde. Furthermore, this has permitted us to bring about still further improvement by making possible the introduction of other desirable components into the bonding composition, especially siliclous and lead components, These new, modified binders prepared as described below show a common characteristic of essential improvement over any previously available in that they fuse completely at temperatures of redness to yield bonded mica plates of superior hardness, adhesion and clarity, and of notably high electrical insulation value.

We may substitute one or more thermally-decomposable boron oxide-forming compounds, such as boric acid, in part or entirely for boron trioxide; such compounds when heated to the manufacturing temperature of the mica plate decompose chemically and leave a residue composed of boron trioxide. In the claims herewith we have, for simplicity, consistently named boron trioxide as typical of the component of the composition which supplies this substance with the intent, however, that such claims shall include within their scope not only boron trioxide itself but also all useful equivalents thereof, for example any thermally-decomposable boron trioxide-forming compound as well as. any desired combination of such materials.

Thus, the improved binders embraced in this invention may be classified into two groups, as follows:

Group 1.-Binders containing boron trioxide with effective proportions of non-elemental fluorine, that is, fluorine in combination, or combined fluorine.

Group 2.Binders containing boron trioxide and effective proportions of combined fluorine, together with a silicious material, or a lead compound, or both a silicious material and a lead compound; these modifications also yielding compositions which are essentially fluxable under the conditions of manufacture to give clear glass-like mica adhesives. For a lead compound, we may substitute the oxide of some other heavy metal such as tin, arsenic, antimony, or bismuth.

In determining the effective proportions of binder components, the thermal behavior, the mechanical properties, and the electrical insulation resistance values, have all been taken into consideration. These criteria have been observed also in the selection of the optimum proportions. Thus, although the tabulated results relate specifically to the effect of the various modifiers of boron trioxide upon the electrical insulation resistances of mica plates bonded with these compositions, we have taken into consideration also their effect in producing improvement in the essential thermal and mechanical properties of the bonding compositions and other heat-resisting substances bonded with them as described above in the objects and subject matter of this application.

The several tables in this application show the percentage compositions of various modified boron oxide formulas and the minimum electrical resistances at 650 C. (1202 F.) of high temperature-resistant mica plates bonded with the fused reaction products of these compositions. The binder formulas are given for the bonding composition mixtures used for application to the mica films in constructing the composite plate, and before fusion. The resistance values represent measurements made on the high temperature-resistant mica plates, bonded with these reaction products that have been produced in situ by heating the composite mica plates containing the unfused compositions to temperatures of redness and subjecting them to adequate applied pressures, and cooling under pressure.

The electrical insulation resistance values given in the tabulations represent measurements made on 10 square inches of the test mica plates, 0.015" in thickness, in a test clamp designed for the purpose. The resistances, which are expressed in megohms at 500 volts D. C. are absolute only for the specific conditions of test and serve, therefore, only as a basis for comparison. The measurements were made during heating to 650 C. (1202 F.) and while the mica plate was kept at 650 C. for 15 minutes, the tabulated values being the minimum resistances measured at'anytime during the test.

In considering these results, itshould be noted that even a relatively small improvement in electrical resistance'is of value, in that it means a substantial decrease in the current leakage of an electrical heater appliance in which the mica plate serves as insulation, thereby reducing shock hazard.

GROUP 1 As stated above, we had found that boron trioxide when used alone as a mica binder failed to fuse thoroughly, under the conditionsof manufacture of high temperature-resistant mica plate, to a clear glass-like adhesive form and that because of the residual opacity, mica platebonded with it exhibited inadequate hardness and mechanical integration. As the result of our investigation, however, we have obtained markedly beneficial results by the addition of effective proportions of hydrofluoric acid, hydrofluosilicic acid, or one or more of their alkali metal or ammonium salts to boron trioxide, and eflecting a probable chemical reaction by fusion. By effective proportions is meant proportions such that the resulting binders shall by virtue of the effect of the added fluorine compound exhibit adequate fluidity and complete fusion to a clear mica-adhesive glass-like form, and by virtue of this clarifying or fluxing effect, the fused bonded mica plates shall possess excellent hardness, adhesion, clarity, and mechanical integration. Depending upon the specific fluorine compound used in the unfused binder composition, these efiective proportions range from about 1% to about 20%.

Another beneficial effect of the addition of fluorine compounds to boron trioxide is in the improvement in electrical insulation resistance values over that shown by boron trioxide alone. Mica plates bonded with compositions containing boron trioxide with various types and percentages of fluorine compounds show improved electrical insulation resistance in every case over plates bonded with boron trioxide alone (see Table I.)

In our tests we used as the sources of the fluorine components one or more of the following: a 52%-55% aqueous solution of hydrofluoric acid; a 30.5% hydrofluosilicic acid solution, and various technical grades of alkali metal and ammonium fluorides and fiuosilicates, (silicofluo rides).

TABLE I Electrical insulation resistances of fused bonded mica plates at 650 C. (1202 F.)

(FORMULAS GWEN FOR BINDER COMPOSITIONS I BEFORE FUSION) The second group of improved binders contains boron trioxide and the necessary effective proportions of combined fluorine as described above,

together with a silicious material, or a lead compound, or both a silicious material and a lead compound, yielding compositions which are essentially fluxable, under the conditions of manufacture, to form clear glass-like mica-adhesive compositions. For the lead compound we may substitute the oxide of some other heavy metal such as tin, arsenic, antimony, or bismuth.

In contrast. to the results tabulated below in Tables 11 and III, our experiments have shown that in the absence of fluorine compounds, boron trioxide combinations with silicious materials or lead compound are incapable of complete fusion to clear permanent glass-like forms under mica plate manufacturing conditions. Similarly, it has not been possible to efiect vitrification of mixtures of silicious materials and lead oxide alone at the temperature of manufacture of high temperatureresistant bonded mica plate. In these cases the lack of fusion and residual opacity produced inadequate hardness, adhesion, clarity, and mechanical integration in the mica plates bonded with these compositions.

We have been able, however, by the addition of eflective proportions of combined fluorine to boron trioxide to flux the silicious materials and lead compounds in these compositions so that mica plates bonded under pressure with the reaction product obtained by fusion in situ exhibit excellent hardness, adhesion, clarity, and mechanical integration, as well as increased electrical insulation resistance and moisture-resistance. Thus, the fluxing efiect of the added fluorine compound is specific and marked.

The three methods described below relate to improved compositions of this group.

First method It has been discovered that from about 1% to about 33% of flnely divided silica or mineral silicates or soluble silicates may be incorporated with boron trioxide to efiect the formation of clear, adhesive, glass-like binders at red heat by the addition of about 1% to about 20% of hydrofluoric acid, hydrofluosilicic acid, or one or more of their alkali metal or ammonium salts. The resulting binders flow under pressure to give clear glass-like adhesives at red heat, so that on cooling, the mica plates bonded therewith are hard, clear, and excellently fused and possess a high degree of mechanical integration and improved high electrical insulation values.

It is our belief that in this type of binder the silicious component is colloidally dispersed in the composition, but whether or not this theory is correct the fact remains that the silicious componentfunctions to maintain and stabilize the fluxing temperature of the mixture at a correct temperature and to enhance materially the electrical insulation resistance of the binder. The

silicious component should have a degree of fineness of at least 300 mesh and should not exceed 33%, of the total weight of the unfused binder components. Greater proportions impart a faint to dense opacity to the bonded mica plate, with consequent inadequacy of the desired mechanical integration. As the silicious component we may elect to use powdered or precipitated silica, preferably silicic acid, or a powdered mineral silicate suchas feldspar, or aqueous solutions (colloidal dispersions) of soluble silicates such as sodium or potassium silicate or metasili'cate. During mixing of the bonding composition the soluble silicates gel in aqueous solutions of boron trioxide, but can be sufllciently dispersed to permit uniform distribution on application.

In Table II, the silicious material in combination with added fluorine compound is shown in every instance to produce increased electrical insulation resistance of mica plates bonded with the fused compositions over that of plates bonded with boron trioxide alone.

TABLE II Electrical insulation resistances of fused bonded mica plates at 650 C. (1202 F.)

(FORMULAS GIVEN FOR BINDER COMPOSITIONS BEFORE FUSION) Boron Elcc. ins

silicious material Fluorine comtrioxide, res. in percent percent pound, percent megnhms 100.0 0. 46 90.0 5. silicic acid 5.0 NHiRHF 'J. 50 81. l 16. 2 slliclc acid 2. 7 KB 1. 40 81. 16. 25 silicic acid 2. 25 HF 1.60 75.0 5. 0 siliclc acid 20. 0 NHiRHF 0. 62 60. 0 33.0 silicic acid 7. 0 N HJRHF 1. 12 81.1 16. 2 iniusorial earth 2.7 NH|F.HF 1.11

Second method We have found that from about 1% to about 20% of a lead compound may be incorporated with boron trioxide to effect the formation of clear, adhesive, glass-like binders at red heat by the addition of about 1% to about 20% of hydrofluoric acid, hydrofluosilicic acid, or one or more of their alkali metal or ammonium salts, so that mica plates bonded with thesetcompositions at fusion temperatures exhibit, on cooling, excellent hardness, adhesion, and clarity, a high degree of mechanical integration, high electrical insulation resistance values, and greater resistance to the action of water vapor.

The specific beneficial efiect of the lead compound is believed to, be to produce greater hardness and increased resistance of-the compositions and products bonded therewith to physical and chemical change upon exposure to moisture. As the lead compound, we prefer to use red lead oxide, but we have found that yellow lead oxide and other lead compounds, such as the nitrate and carbonate, may be used. In place of the individual lead and fluorine compounds, we may use lead fluorine or lead silicoiiuoride. We have found also that the oxides of other heavy metals, such as arsenic, bismuth, tin, and antimony, may be used in place of the lead compound, but in relatively smaller proportions.

Table III shows the improved electrical insulation resistances at 650 C. (1202 F.) of fused mica plates bonded with compositions of these types, over that of plates bonded with boron trioxide alone.

Tum: III

Electrical insulation resistances of fused bonded mica plates at 650 C. (1202 F.)

(FORMULAS GIVEN FOR BINDER COMPOSITIONS BEFORE FUSION) Third method We have discovered that mixtures comprising from about 50% to about 97% of boron trioxide, from about 1% to about 20% of a fluorine compound, from about 1% to about 40% of a silicious material, and from about 1% to about 20% of a lead compound yield compositions which produce still further improvement. In this type also the silicious material and lead compound are essentially fluxable, under the conditions of manufacture, the compositions yielding glass-like micaadhesives, and mica plates bonded therewith exhibit increased hardness, adhesion, clarity, and mechanical integration, improved electrical insulation resistance, and greater resistance to physical and chemical change upon exposure to moisture.

It is our belief that in this type of binder the silicious material and the lead compound are reacted chemically in the composition at the fusion temperature of mica plate manufacture to produce, at least to some extent, a true lead glass, as compared with the binder described under the first method, in which the silicious component is believed to be colloidally dispersed. For this reason, such a high degree of fineness as 300 mesh is not required of the silicious material in this type, and higher proportions may be used. Reference has been made earlier in this specification to the silicious materials, lead compounds and fluorine compounds that may be used in these compositions.

This method incorporates not only the specific effects of improvement of the individual boron trioxide modifiers as they have been described above, but also produces in the composition a collective effect of still greater improvement in increased hardness and moisture-resistance.

Table IV shows the improvement in electrical insulation resistance of fused mica plates bonded with various compositions of this type over that of plates bonded with boron trioxide alone.

TABLE "IV Electrical insulation resistances of fused bonded mica plates at 650 C. (1202 F.)

(FORMULAS GIVEN FOR BINDER COMPOSITIONS BEFORE FUSION) The percentages given in the several tables are not to be taken as specific limitations, but rather to demonstrate the range of or imum proportion. Higher and lower proportions in the ranges given for the various components have also yielded effective improvement.

Although the invention has been described with particular emphasis upon the use of these compositions as binders for mica films, it is readily apparent that their usefulness may be extended to various types of insulation materials, and to their service as fused glass-like materials.

The preceding description relates to the preferred embodiments of the invention. M nor changes in details or combinations with suitable other binders are intended to be included in the spirit and scope of the invention.

We claim:

1. The method of making a composite bonded product which comprises associating discrete matter with an unfused composition containing boron trioxide and a fluorine-containing compound, subjecting said associated discrete matter and composition to a temperature sufllcient to effect thermal reaction of the components of" said composition and to obtain a fused reaction product capable of and bonding said discrete matter under pressure into an integral body, so bonding said discrete matter, and cooling the bonded product while under pressure.

2. The method according to claim 1 wherein said unfused composition also contains at least one agent, capable of modifying the boron trioxide component, selected from the group consisting of a silicious material and a heavy metaloxygen compound.

3. The method of making a composite bonded product which comprises associating mica films with an unfused composition containingfrom about 50% and upwards to about 97% of boron trioxide and from about 1% upwards to about 20% of a fluorine-containing compound, subjecting said associated mica films and composition to a temperature suflicient to effect thermal reaction of the components of said composition and to obtain a fused reaction product capable of and bonding said mica films into an integral body, so bonding said mica films, and cooling the bonded product while under pressure.

4. The method according to claim 3 wherein said unfused composition also contains at least one agent capable of modifying the boron trioxide selected from the group consisting of from about 1% upwards to about 40% of a silicious material, and from about 1% to about 20% of a heavy metal-oxygen compound.

5. A composite mica product composed of a plurality of layers of mica films integrated with a binder comprising boron trioxide fused with a fluorine-containing compound.

6. A composite mica product in accordance with claim 5, in which the fluorine-containing com pound is selected from the group consisting of hydrofluoric acid, an alkali metal salt of hydrofluoric acid and an ammonium salt of hydrofluoric acid.

7. A composite mica product in accordance with claim 5, in which the fluorine-containing compound is selected from the group consisting of hydrofluosilicic acid, an alkali metal salt of hydrofiuosilicic acid and an ammonium salt of hydrofiuosilicic acid.

8. A composite mica product in accordance with claim 5, in which the fluorine-containing compound is ammonium acid fluoride.

9. A composite mica product composed of a plurality of layers of mica films integrated with a binder comprising boron trioxide fused with a fluorine-containing compound and a silicious material.

10. A composite mica product in accordance with claim 9, in which the silicious material is selected from the group consisting of silica, silicic acid, mineral silicates, water-soluble silicates and metasilicates.

11. A composite mica product in accordance with claim 9, in which the silicious material is silicic acid.

12. A composite mica product in accordance with claim 9, in which the fluorine-containing compound is ammonium acid fluoride and the silicious material is silicic acid.

13. A composite mica product composed of a plurality of layers of mica films integrated with a binder comprising boron trioxide fused with a fluorine-containing compound and a heavy metal-oxygen compound.

14. A composite mica productin accordance with claim 13, in which the heavy metal-oxygen compound is selected from the group consisting of the oxides of lead, tin, arsenic, antimony and bismuth.

15. A composite mica product in accordance with claim 13, in which the heavy metal-oxygen compound is red lead oxide.

16. A composite mica product in accordance with claim 13, in which the fluorine-containing compound is ammonium acid fluoride and the heavy metal-oxygen compound is red lead oxide.

17. A composite mica product composed of a plurality of layers of mica films integrated with a binder comprising boron trioxide fused with a fluorine-containing compound, a silicious material and a heavy metal-oxygen compound.

18. A composite mica product in accordance with claim 17, in which the fluorine-containing compound is ammonium acid fluoride, the silicious material is silicic acid and the heavy metaloxygen compound is red lead oxide.

19. A laminated mica product composed of mica films adhered together with a bonding agent comprising the fused reaction product of a composition containing between from about 80% to about 99% of boron trioxide, and between from about 1% to about of a fluorine-containing compound.

20. A laminated mica product composed of mica fllms adhered together with a bonding agent comprising the fused reaction product of a composition containing between irom about to about 98% boron trioxide, between from about 1% to about 20% of a fluorine-containing compound and between from about 1% to about 33% of a silicious material.

21. A laminated mica product composed of mica films adhered together with a bonding agent comprising the fused reaction product of a composition containing between from about to about 98% boron trioxide, between from about 1% to about 20% of a fluorine-containing compound, and between from about 1% to about 20% of a heavy metal-oxygen compound.

22. A laminated mica product composed of mica films adhered together with a bonding agent comprising the fused reaction product 01' a composition containing between from about 50% to about 97% boron trioxide, between from about 1% to about 20% of a. fluorine-containing compound, between from about 1% to about 40% of a sili cious material, and between from about 1% to about 20% of a heavy metal-oxygen compound.

WILLIS A. BOUGHTON. WILLIAM R. MANSFIELD.

CERTIFICATE OF CORRECTION.

Patent No. 2,196,972. April 16, 191m.

WILLIS A; BOUGHTON, ET AL.

It is hereby certified-that error appears in the printed specification of the above numbered patent requiring correction as follows; Page 2, first column, line 66, for the word "composition" readcompositi0ns, page LL, first column, line 15, for "compound" read compounds-; page 5, second column,. line 58, claim 14., after "1%" insert -upwards--; and that'the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 18th day of June, A. D. 191w.

Henry Van Arsdale,

(Seal) Acting Commissioner of Patents. 

